feat(teleport B): PhysicsEngine.IsLandblockTerrainResident worldReady query

Adds a high-16-bit-prefix landblock residency check used as the teleport
worldReady gate — true once the destination landblock's terrain+cells
have been registered via AddLandblock, regardless of whether the caller
passes a canonical (0xFFFF), cell-resolved, or bare landblock id.

Two TDD tests confirm: false before registration, true after, and
that a cell-resolved id on the same landblock returns true.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -31,6 +31,20 @@ public sealed class PhysicsEngine
     /// <summary>Number of registered landblocks (diagnostic).</summary>
     public int LandblockCount => _landblocks.Count;
 
+    /// <summary>
+    /// True once the landblock covering <paramref name="cellOrLandblockId"/> has had its
+    /// terrain + cells registered via <see cref="AddLandblock"/>. Accepts a canonical
+    /// (0xFFFF) id, a cell-resolved id, or a bare landblock id — compares on the high 16
+    /// bits. This is the teleport "worldReady" gate (the destination is grounded).
+    /// </summary>
+    public bool IsLandblockTerrainResident(uint cellOrLandblockId)
+    {
+        uint prefix = cellOrLandblockId & 0xFFFF0000u;
+        foreach (var key in _landblocks.Keys)
+            if ((key & 0xFFFF0000u) == prefix) return true;
+        return false;
+    }
+
     /// <summary>
     /// Cell-based spatial index for static object collision.
     /// Populated during landblock streaming; queried by the Transition system.